1. Field of the Invention
The present invention relates to conduits and their methods of manufacture and use in the conveyance of fluids and gases. More particularly, the present invention relates to flexible conduits that are submergible in deep water to be employed to convey high pressure fluids with provision for venting high pressure gases trapped in the layers forming the conduit wall.
2. Description of the Prior Art
Flexible conduits are commonly employed to convey liquids and gases between submerged pipelines and offshore oil and gas production facilities and other installations. These conduits are subjected to high internal and external pressures, as well as chemical actions associated with the seawater surrounding the submerged conduits and the fluids being transported within the conduits.
The conduits are customarily constructed of axially extending, overlapping tubular layers of strengthening and sealing materials that are individually designed to provide specific structural and barrier protection against the pressure force and chemical hazards encountered in the conduit use. Typical conduit construction includes a helically wound metal band with adjacent bands interlocked that provides protection against external hydrostatic pressure and radial burst forces acting from within the conduit. Additional strengthening layers of flat wire bands or stranded materials and other materials with high tensile strength are wound in alternating layers about the metal band layer to provide further burst strength and to increase the axial strength of the conduit. Elastomeric layers of polymetric material are extruded over the wound layers at appropriate intervals to provide a sealing layer or barrier seal for preventing radial fluid and pressure migration through the conduit wall.
Entry of gas between the sealing layers of the conduit wall can lead to damage in these multilayered conduits. The danger of such damage becomes particularly likely where the conduit is used to convey high pressure fluids or gases in deep water. High pressure gas in the conveyed fluid can permeate through a sealing layer of the elastomeric material and become trapped between radially spaced sealing layers of the conduit. Gas trapped between the walls of the conduit expands when the external pressure on the conduit is lowered, as, for example, when the submerged conduit is raised to the water surface or when the internal bore pressure is lowered, as, for example, when the conduit is shut in. Where a sealing layer is interposed between trapped gas and strengthening material of limited burst or collapse resistance, a pressure differential across the sealing layer caused by the expanding trapped gas can produce radial forces sufficient to rupture, collapse, or otherwise damage the strengthening materials.
The problem of expanding gases in flexible conduits is primarily limited to applications where the conduit is submerged in relatively deep water and is employed to transport high pressure gases or fluids containing dissolved gases. Such situations exist with many oil and gas operations offshore. The problem is not present when the conveyed fluid is a liquid that is free of any dissolved gas.
One attempt to address the problem of trapped gas in the layers of flexible piping has been to simply increase the strength of the strengthening and sealing layers to withstand the maximum possible pressure differential that may occur from the trapped gases. The solution is expensive, and the more resistant the conduit is made to the pressure differentials, the larger and stiffer the conduit becomes.
The annulus between sealing layers in the end segment of a submerged conduit line is commonly vented through the riser pipe to the water surface. Longer lengths of conduits that are joined in sections vent the trapped gas in the submerged sections directly into the water adjacent the connector. The trapped gas is vented through a one-way valve into the surrounding water. A problem with the systems of this type is that the gas vents at the hydrostatic pressure of the water into which it vents. In deep water, the pressure may be substantial. Moreover, failure of the valve to properly vent the trapped gas as the conduit is retrieved to the surface allows the gas to expand and damage the conduit when the conduit is retrieved to the water surface or build compressive forces sufficient to collapse the conduits underlying layers when the conduit bore pressure is lowered. Such valve failures are difficult to prevent in applications where the valve is exposed to the harsh sea bottom environment for long periods of time. Proper operation of the valve is also difficult to test when the conduit is submerged.